This application requests funding for experiments that form part of an ambitious long term project to study structure-function relationships in alphaviral and flaviviral structural and nonstructural proteins, to be undertaken in collaboration with the structural biology group at the Purdue University. Alphaviruses and flaviviruses are two genera of plus strand RNA viruses each of which contains many significant human and animal pathogens. These pathogens include yellow fever virus, dengue virus, West Nile virus, St. Louis and Japanese encephalitis viruses, and tick-borne encephalitis virus among the flaviviruses, and Eastern, Western, and Venezuelan equine encephalitis viruses, Ross River virus, Mayaro virus, and chikungunya virus among the alphaviruses. As a long term goal, structures of intact alphaviruses and flaviviruses as well as structures of all of the structural and nonstructural proteins of a number of alphaviruses and flaviviruses will be obtained. The functions of the structures obtained in this way will be probed using molecular genetic techniques in which the effects of previously known mutations as well as new mutations introduced into the proteins will be analyzed in light of structure. The current proposal focuses on the structures of intact alphaviruses and flaviviruses and the structures of proteins E1 and E2 of alphaviruses, the helicase and protease of alphaviruses, NS3 of alphaviruses, and protein E of flaviviruses.